Prolonged tack toners prepared from thermoplastic resins and a plasticizer are well known in the art. Upon heating the thermoplastic resin and plasticizer mixture, the mixture becomes tacky or sticky and remains tacky for a considerable time even after cooling. This property is known as delayed tack or setting and has been found useful in the preparation of adhesive compositions, as well as in thermography, to produce a master which is imagewise exposed by means of heat, toned, and the toned image is then transferred onto an image receptor to provide a copy of the original image. In the above-described process, the prolonged tack property is present in the form of a continuous film over an imagewise tacky element.
The prolonged tack toners and transfer process described above can be used to make four-color proofs. For example, in preparing a four-color proof consisting of cyan, magenta, yellow, and black, the cyan toner image is transferred to a receptor. The magenta toner image is then transferred to the same receptor, with the two images in register. The yellow image is then transferred and finally, the black image is transferred so that all four images are in register. However, if the transferred toned image on the receptor remains tacky when the next color is transferred, the image on the receptor can back transfer to the surface of the photopolymer surface of the next color, resulting in partial image loss from the receptor to the photopolymer. Back transfer problems and the resulting image loss are unacceptable in the printing industry.
U.S. Pat. No. 4,461,823, issued to Held on Jul. 24, 1984, describes multiple transfer of tacky image areas using prolonged tack toners wherein the toners can be prepared by combining a thermoplastic resin, e.g., polystyrene, with a plasticizer or a copolymer of methylmethacrylate (90)/methacrylic acid (10) and triphenyl phosphate plasticizer. A dye or colored pigment can be combined with the resin/plasticizer component. One of the main disadvantages with these toners is that it is difficult to transfer a toned image to an image receptor at low temperatures without getting some back transfer when transferring the next color.
U.S. Pat. No. 4,469,625, issued to Held on Sep. 4, 1984, describes prolonged tack toners for the preparation of electric circuits. Examples of organic polymers and plasticizers are described in column 1 and include polystyrene and N-cyclohexyl-p-toluenesulfonamide, poly(methylmethacrylate) (90)/methacrylic acid (10) and triphenyl phosphate. One of the main disadvantages using these toners is that it is difficult to transfer a sufficient amount of toner to an image receptor at a low temperature. Applicant's assignee's copending application, Ser. No. 07/422,361 filed Oct. 16, 1989, issued Aug. 13, 1991, as U.S. Pat. No. 5,039,588, describes a non-electroscopic prolonged tack toner comprising an organic, thermoplastic terpolymer, a solid plasticizer and optionally a colorant wherein said toner is suitable for transferring toned images at low temperatures to an image receptor. The toners can be used to make a four-color proof. The toned image is transferred to a receptor. However, on subsequent transfers back transfer of the previously transferred image is observed.
U.S. Pat. No. 4,869,996 issued to McCartin et al. on Sep. 26, 1989, describes an improved process for preparing negative images from a positive-type photosensitive element. The process utilizes two separate applications of contrasting toning materials in order to produce a negative image. Each application of toning material is carried out at a different temperature. The process can also be used to produce positive images as well as negative multi-layer, multi-color images.